SV40 DNA replication can be divided into four stages: 1) initiation of DNA replication, 2) propagation of the replication fork, 3) segregation of the newly synthesized replicative intermediates and 4) maturation of the segregated progeny DNA by a gap filling process. These stages of DNA replication are being studied by an in vitro permeabilized cell system. The SV40 A gene product is required for the initiation of each round of DNA replication and assays are being developed to look at gene A product function in vitro. A protein kinase activity that phosphorylates or exchanges phosphate groups from this protein with gamma-labeled ATP has been isolated and is being purified. The association of this A gene product with viral and cellular chromatin is under study so as to understand the protein-chromatin interactions and develop ways to initiate DNA synthesis in vitro. The SV40 A gene product is being purified by several methods and tested for possible function and activity.